bake: test secrets override

[crazy-max]

relates to docker/bake-action#285

[crazy-max]

It doesn't append: https://github.com/docker/buildx/actions/runs/12768837294/job/35590048834?pr=2912#step:6:1346

 === FAIL: bake TestReadTargets/SecretsOverride (0.00s)
    bake_test.go:161: 
        	Error Trace:	/home/runner/work/buildx/buildx/bake/bake_test.go:161
        	Error:      	"[id=BAR,env=BAR]" should have 2 item(s), but has 1
        	Test:       	TestReadTargets/SecretsOverride
    --- FAIL: TestReadTargets/SecretsOverride (0.00s)

Looking at

buildx/bake/bake.go

Lines 928 to 933 in 1eb167a

	case "secrets":
	secrets, err := parseArrValue[buildflags.Secret](o.ArrValue)
	if err != nil {
	return errors.Wrap(err, "invalid value for outputs")
	}
	t.Secrets = secrets

it seems expected